One of the implicit conditions for the UAV-BS deployment is that the backhaul rate should be high enough to support the traffic of the UAV-BSs [1]}. To satisfy this requirement, the energy-efficiency of the backhaul source should also be considered [2]}, [3]}. However, providing backhaul links directly from the ground source is inefficient owing to potential blockages, which generate non-LoS (NLoS) links and significantly degrade the system's energy-efficiency. This gives rise to a critical issue for future wireless networks with higher frequencies and wider bandwidths [4]}, [5]}, [6]}, [7]}, [8]}.
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